I wish GitHub would use tar.xz instead of tar.gz for source code downloads. Mozilla's Fira source code with tar.gz is 77.1 MB while with tar.xz is 66.3 MB. That's about 11 seconds faster download on an average internet connection.
I think the big concern isn’t size-over-the-wire as much as decompression speed. GitHub repos get used by several package managers for fetching source and doing local compiling. For embedded devices that might be on slow ARM CPUs download speed differences measured in seconds are a positive trade-off for decompression differences measured in minutes.
As a "standard" user I never bother to check what was the best or few bests file formats when it comes to compression... Using the Zip format is like a reflex. There are so many other formats I would not know what to use...
The standard Zip archive has 89.7 MB. It's 23.4 MB bigger. That's almost half of XZ archive.